When fluid flows past an obstacle, the obstacle causes a disturbance in the fluid flow. This disturbance is manifested by a vortex generated on one side of the obstacle followed shortly thereafter by another vortex generated on the other side of the obstacle. The two sides of the obstacle continue to alternately generate, or shed, vortices so long as the fluid continues to flow. The frequency at which the two sides of the obstacle shed these vortices is proportional to the velocity of the fluid relative to the obstacle. It is this phenomenon that is the basis for the operation of the known vortex flowmeter.
In a vortex flowmeter, an obstacle in the fluid flow, generally a bluff body, generates an alternating series of vortices. These vortices flow past a pressure transducer at or near the bluff body. Since each vortex is associated with a low pressure zone in the fluid, each time a vortex flows past the pressure transducer, it causes the pressure transducer to generate a pulse having an amplitude proportional to the fluid density and to the square of the fluid velocity. Since the vortices flow with the fluid, the frequency of these pressure pulses is proportional to the fluid velocity. The signal generated by the pressure transducer thus includes a fundamental frequency corresponding to the fluid velocity.
In addition to information about the fluid velocity, the signal generated by the pressure transducer also contains low-frequency components corresponding to other disturbances, such as vibrations from motors, pumps, or unsupported sections of the pipe through which the fluid flows. The transducer signal can also contain high frequency components from other acoustic sources, such as loud noises in the room through which the pipe flows. Additional signal components, both high and low frequency, can also arise from fluid turbulence within the pipe.
These extraneous signal components, collectively referred to as "noise", are generally filtered out by a bandpass filter having a center frequency at or near the vortex shedding frequency. However, the fact that the vortex shedding frequency is unknown and constantly changing seriously hampers the ease with which one can tune a bandpass filter to that frequency. This difficulty is addressed by the adaptive bandpass filter disclosed in Vignos, U.S. Pat. No. 5,576,497, "Adaptive Filtering for a Vortex Flowmeter," which is incorporated herein by this reference.
The noise components rejected by the adaptive bandpass filter are not, however, without some value. For example, subtle changes in the spectrum of the noise generated by a pump or motor can foreshadow an imminent mechanical breakdown. Because the pressure signal is responsive to fluid density, changes in the spectrum of the pressure signal can indicate an undesirable change in the composition of the fluid flowing through the pipe. It is therefore useful to monitor the noise components rejected by the adaptive bandpass filter.
In order to adjust the filter passband to match the changing fluid velocity, the adaptive bandpass filter disclosed in Vignos continuously monitors the pressure signal. If the adaptive bandpass filter "looks away," it is apt to lose track of the fluid velocity and to be unable to recover for some time. As a result, it is impractical to attempt time division multiplexing of the signal from the pressure transducer to the adaptive bandpass filter.
One known approach to observing the noise spectrum is to connect a sweep filter analyzer or similar device directly to the pressure transducer, in parallel with the adaptive bandpass filter. This, however, is a cumbersome procedure since it requires a separate connection at the transducer, an additional piece of hardware, and significant additional power consumption.
What is therefore desirable in the art is a system that can simultaneously track the velocity component of the pressure transducer signal and observe the noise components of that signal.